update targeting for Promise.prototype.finally

[JakeChampion]

MDN browser support table -- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/finally#Browser_compatibility

This config is a bit hard to understand because we configure it to only target browser which have a native Promise but not a native Promise.prototype.finally, this is because browsers which do not have a native Promise will get our Promise polyfill which already includes Promise.prototype.finally